1. Field of the Invention
This invention pertains to non-destructive testing of manufactured parts and more specifically, use of projection imaging in non-destructive testing.
2. Description of Related Art
In the manufacturing of parts, an engineer creates a design or blueprint of locations and shapes of structures of the part identified in part coordinate system. These may be designed on a CAD/CAM workstation. Machines manufacture the part according to the design, or CAD model, but mechanical error causes differences between the manufactured part and the theoretical design (CAD model). In order to insure proper quality, the manufactured parts are examined, typically by non-destructive testing to insure accurate construction. Those which fail are either re-manufactured or discarded.
A typical type of non-destructive testing involves imaging of the part such that internal structures are visible. One such type of imaging is X-ray imaging, employing an X-ray imaging device which creates 2D projection images of the 3-D part. This usually requires calibration of the imaging device.
Geometric calibration of an imaging device refers to the ability to map the 3-D world to an image plane of the image device. If the geometric calibration of a imaging device is known, one can locate in the image any arbitrary point in space.
Two methods for geometrically calibrating an imaging device, and in general, any imaging setup, are prevalent in non-destructive testing.
In the first method, one explicitly measures the various parameters that determine the mapping from the 3-D world to the 2-D image plane. For example, the focal length, the distance from the source to the center of the part, the source to detector array distance, the detector spacing, the spacing between image lines, etc. are measured. Knowing these parameters, any point of the object space can be projected into the image. This method will be referred to as the "direct calibration".
Direct calibration suffers from the following disadvantages.
1. The calibration is done once and frozen till the next calibration run, so it does not reflect any changes in imaging parameters from one image acquisition to the next. One has to do direct calibration often enough in order to minimize the effects of changes in the calibrated parameters between acquisitions.
2. In many manufacturing situations, in-situ calibration is essential because the setup is not repeatable. In these cases direct calibration cannot be used.
3. The measured parameters may fluctuate from image to image.
In another calibration method, specially designed, precisely calibrated parts or patterns are imaged to either deduce the imaging parameters or to adjust the imaging device to a known calibration. For example, one might image a pattern of parallel wires with decreasing spacing to determine the magnification and the resolution of the imager. This calibration method also suffers from all three disadvantages mentioned above.
Typically, a part is examined by placing it in a fixture, known as a `six point nest`, having reference structures located at specific known points on the fixture. The distance from each of 6 preselected points on the surface of the part to the reference structures is determined. These distances are compared to preset tolerance distances to determine if the part is to be rejected.
The use of a six-point nest to orient a part to be examined is commonplace. Sometimes, the part is machined, while in its six-point nest, to provide alternate places to hold the part while retaining the same location relative to the fixture.
In order to deduce the orientation of the part in an X-ray image, locations of the fixture and visible features on the part are essential.
Another method of non-destructive testing compared the CAD model with a 3D model constructed from a number of projection images.